Electrochemical cell for lithium accumulator comprising a specific negative electrode made of metallic lithium and a positive electrode on aluminium collector

ABSTRACT

An electrochemical cell for a lithium accumulator comprising: a negative electrode comprising metallic lithium as active material; a positive electrode associated with an aluminium current collector; and an electrolyte placed between the negative electrode and the positive electrode, wherein the negative electrode is provided with a layer comprising a compound containing aluminium at its face in contact with the electrolyte, and in that the electrolyte comprises at least one lithium salt chosen from among lithium imidide, lithium triflate, lithium perchlorate salts and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from French Patent Application No. 1900047 filed on Jan. 3, 2019. The content of this application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates to an electrochemical cell for a lithiumaccumulator comprising an association between a specific negativeelectrode based on metallic lithium and a positive electrode on analuminium current collector that in particular can obtain excellentcyclability performances.

The general field of the invention can thus be defined as energy storagedevices, particularly lithium electrochemical accumulators.

Energy storage devices are conventionally electrochemical accumulatorsfunctioning on the principle of electrochemical cells capable ofoutputting an electric current due to the presence in each of them of apair of electrodes (a positive electrode and a negative electroderespectively) separated by an electrolyte, the electrodes comprisingspecific materials capable of reacting according to anoxidation-reduction reaction, through which electrons are producedgenerating an electric current and the production of ions that willcirculate from one electrode to the other through an electrolyte.

The most frequently used accumulators of this type are:

-   -   Ni-MH accumulators using metal hydride and nickel oxyhydroxide        as electrode materials;    -   Ni-Cd accumulators using cadmium and nickel oxyhydroxide as        electrode materials;    -   Acid-Lead accumulators using lead and lead oxide PbO₂ as        electrode materials; and    -   lithium accumulators such as lithium ion accumulators (in which        in principle, the positive electrode active material and the        negative electrode active material are lithium insertion        materials) or lithium-metal accumulators (of which the negative        electrode is composed of metallic lithium).

Since lithium is a particularly lightweight solid element and has thelowest electrochemical potential, thus enabling access to an interestingenergy density per unit mass, the lithium accumulators mentioned abovelargely replaced the other accumulators mentioned above and are found tobe particularly interesting for fields in which endurance is anessential criterion, as is the case in computer, video, mobile telephonyfields, in transport such as electric vehicles, hybrid vehicles, or inthe medical, space and microelectronic fields.

Lithium-metal accumulators conventionally comprise at least oneelectrochemical cell composed of the following elements:

-   a negative electrode based on metallic lithium;-   a positive electrode based on a compound capable of inserting    lithium; and-   an electrolytic membrane (that can also be called a separator)    located between said negative electrode and said positive electrode.

However, although the metallic lithium used to form the negativeelectrodes can obtain a high nominal cell voltage and excellent energydensities per unit mass and per unit volume, it is neverthelessdisadvantageous during accumulator cycling processes. During cyclingprocesses, metallic lithium is alternately dissolved in the electrolytein the form of ions during discharge, and redeposited on the negativeelectrode during recharge. At the end of a certain number of cycles,particularly when the electrolytic membrane comprises a liquidelectrolyte, lithium dendrites can form that can firstly contribute todenaturing the physical integrity of the membrane and secondly cangenerate a short circuit phenomenon when the dendrites physicallyconnect the negative electrode to the positive electrode, which canresult in particular in a substantial rise in the battery temperatureand its irreversible degradation. Furthermore, the metallic lithium canreact with a liquid electrolyte to form partially inactive lithium foam,that entrains a loss of active material during cycling thus inducing alimited life of accumulators functioning with metallic lithium at thenegative electrode.

To prevent the formation of foam and dendrites, it has been proposed onthe market lithium accumulators for which the negative electrode isbased on metallic lithium and using a solid electrolyte to create aseparation between the negative electrode and the positive electrode,and more specifically a polymer electrolyte comprising polyoxyethylene(known under the abbreviation POE). This electrolyte is less reactive tometallic lithium than liquid or gel electrolytes, and is also used inthe form of a thin layer, which requires lower current densities perunit area and thus reduces the risk of the formation of dendrites.However, this technology requires a high operating temperature(particularly more than 50° C.) to compensate for the insufficientconductivity of this solid electrolyte at ambient temperature.

In order to be able to benefit from properties inherent to the use ofmetallic lithium (particularly in terms of the density per unit mass andper unit volume) while avoiding the disadvantages related to the use ofa polymer electrolyte, the authors of this invention fixed themselvesthe objective of proposing an electrochemical cell for a lithiumaccumulator using a metallic lithium electrode as the negativeelectrode, while limiting the phenomenon of formation of lithium foamand dendrites and obtaining good cyclability properties.

Presentation of the Invention

Thus, the invention relates to an electrochemical cell for a lithiumaccumulator comprising:

-   a negative electrode comprising metallic lithium as active material;-   a positive electrode associated with an aluminium current collector;    and-   an electrolyte placed between said negative electrode and said    positive electrode, characterised in that the negative electrode is    provided at its face in contact with the electrolyte with a layer    comprising a compound containing aluminium, and in that the    electrolyte comprises at least one lithium salt chosen from among    lithium imidide, lithium triflate, lithium perchlorate salts and    mixtures thereof.

The authors of this invention have been able to demonstrate excellentcyclability properties due to the presence of this layer on the surfaceof the negative electrode, that can be explained particularly by theprotective role performed by this layer with regard to the foamformation and dendrites problem.

Before going into the description of this invention in further detail,we will give the following definitions.

In the above and in the following description, negative electrodeclassically means the electrode that acts as the anode when the batteryoutputs current (in other words when it is in the discharge process) andthat acts as the cathode when the battery is in the charge process.

In the above and in the following description, positive electrodeclassically means the electrode that acts as the cathode when thebattery outputs current (in other words when it is in the dischargeprocess) and that acts as the anode when the battery is in the chargeprocess.

According to the invention, the negative electrode comprises metalliclithium as active material. In particular, it may be composedexclusively of metallic lithium (which in other words means that it doesnot comprise any ingredients other than metallic lithium). It can alsobe associated with a current collector and more specifically, a coppercurrent collector.

From a structural point of view, the negative electrode may be in theform of a 5 μm to 500 μm thick lithium plate or lithium sheet.

The positive electrode may preferably comprise an active material chosenfrom among lithiated phosphates comprising at least one metallictransition element, the lithiated oxides comprising at least onemetallic element (for example at least one metallic transition elementand/or at least one metallic post-transition element), lithium basedmetal sulfides, vanadium oxides, disulfides based on at least onemetallic transition element, elementary sulfur and mixtures thereof.

Among lithiated phosphates comprising at least one metallic transitionelement, mention may be made of lithiated phosphates with formulaLiM¹PO₄, wherein M¹ is chosen from among Fe, Mn, Ni, Co and mixturesthereof, such as LiFePO₄, LiMnPO₄.

Among lithiated oxides comprising at least one metallic element, mentionmay be made of simple oxides or mixed oxides (in other words oxidescontaining several distinct metallic transition elements) comprising atleast one metallic element, such as oxides containing nickel, cobalt,manganese and/or aluminium (these oxides possibly being mixed oxides).

More specifically, among mixed oxides containing nickel, cobalt,manganese and/or aluminium, mention can be made of compounds with thefollowing formula:

Li_(1+x)M²O₂,

wherein M² is an element chosen from among Ni, Co, Mn, Al and mixturesthereof and x is greater than or equal to 0.

Among examples of such oxides, mention may be made of lithiated oxidesLiCoO₂, LiNiO₂ and mixed oxides Li(Ni,Co,Mn)O₂ such asLi(Ni_(1/3)Mn_(1/3)CO_(1/3))O₂, Li(Ni_(0,8)Mn_(0,1)Co_(0,1))O₂ orLi(Ni_(0,6)Mn_(0,2)Co_(0,2))O₂ (also known under the name NMC)),Li(Ni,Co,Al)O₂ (such as _(Li)(N_(0,8)Co_(0,15)Al_(0,05))O₂ also knownunder the name NCA) or Li(Ni,Co,Mn,Al)O₂, oxides said to be lithium-richor overlithiated oxides, such as oxides of Li_(1+x)(Ni,Co,Mn)O₂, inwhich x is greater than 0.

Among oxides comprising nickel, cobalt, manganese and/or aluminium,mention may also be made of oxides with a spinel structure such asLiMn₂O₄.

Among lithium based metal sulfides, mention may advantageously be madeof “rock-salt” type metal sulfides and particularly compounds withformula Li₂TiS₃ or Li₃NbS₄.

Among vanadium oxides, mention may be made in particular of vanadiumoxide V₂O₅.

Among disulfides based on at least one metallic transition element,mention may be made in particular of disulfides crystallising in apyrite type crystalline type structure, such as FeS₂, TiS₂.

Apart from the presence of an active material, the positive electrodemay comprise an organic binder, in particular a polymeric binder such aspolyvinylidene fluoride (known under the abbreviation PVDF), acarboxymethylcellulose mixture (known under the abbreviation CMC) with astyrene-butadiene type latex (known under the abbreviation SBR) or withpolyacrylic acid (known under the abbreviation PAA) and one or severalelectricity conducting additives that can be carbon materials such ascarbon black.

Also from a structural point of view, the positive electrode may be inthe form of a composite material comprising a matrix of polymericbinder(s) such as PVDF (for example with a content of 1 to 10% by massrelative to the total mass of the electrode) within which fillerscomposed of the active material are dispersed (for example with acontent of 80 to 98% by mass relative to the total mass of theelectrode) and possibly the electricity conducting additive(s) such ascarbon black (for example with a content of 1 to 8% by mass relative tothe total mass of the electrode), said composite material beingdeposited on an aluminium current collector.

The cells according to this invention also comprise an electrolytelocated between the positive electrode and the negative electrode, thesurface of the negative electrode in contact with the electrolyte beingprovided with a layer comprising a compound comprising aluminium and theelectrolyte comprising at least one lithium salt chosen from amonglithium imidide, lithium triflate, lithium perchlorate salts andmixtures thereof.

More specifically, the electrolyte may be a liquid electrolyte which,apart from the above-mentioned lithium salt(s), may comprise one orseveral organic solvents, the organic solvents possibly being:

-   solvents in the carbonate solvents family such as cyclic carbonate    solvents, linear carbonate solvents and mixtures thereof;-   solvents in the ethers family, such as 1,3-dioxolane,    tetraethyleneglycoldimethylether, 1,2-dimethoxyethane; and mixtures    thereof.

The electrolyte may also be a gel electrolyte, for example a liquidelectrolyte like that explained above, impregnating a polymer matrix(for example an polyethylene oxide matrix (also designated by theabbreviation PEO), a polyacrylonitrile matrix (also designated by theabbreviation PAN), a polymethyl methacrylate (also designated by theabbreviation PMMA) or a polyvinylidene fluoride (also designated by theabbreviation PVDF), that gelifies in contact with this liquid adhesive.

As examples of cyclic carbonate solvents, mention may be made ofethylene carbonate (symbolised by the abbreviation EC), or propylenecarbonate (symbolised by the abbreviation PC).

As examples of linear carbonate solvents, mention may be made ofdimethyl carbonate (symbolised by the abbreviation DMC), diethylcarbonate (symbolised by the abbreviation DEC), or ethylmethyl carbonate(symbolised by the abbreviation EMC).

In particular, the liquid electrolyte may comprise a mixture ofcarbonate solvents, for example an EC/DMC mixture, for example in equalquantities.

When the lithium salts are lithium imidide salts, they can be chosen inparticular from among lithium imidide salts such as lithiumbis(trifluoromethane)sulfonylimidide with formula Li[N(SO₂CF₃)₂) (knownunder the abbreviation LiTFSI), lithium bis(fluorosulfonyl)imidide withformula Li[N(SO₂F)₂] (known under the abbreviation LiFSI) and lithiumbis(pentafluoroethane)sulfonylimidide with formula Li[N(SO₂CF₂CF₃)₂](known under the abbreviation LiBETI).

Furthermore, when the electrolyte is a liquid electrolyte, in otherwords it is not a gel, it may imbibe a separating element, for example aporous polymeric separating element placed between two electrodes of theaccumulator, it being understood that the polymer(s) forming theseparator do not gel, unlike the case with gelled electrodes.

In particular, advantageous electrochemical cells conforming with theinvention include the following positive electrode lithium salt/activeelectrode combinations:

-   when the lithium salt is LiTFSI, the positive electrode    advantageously comprises, as active material, a lithiated phosphate    comprising at least one metallic transition element such as LiFePO₄    and/or a disulfide based on at least one metallic transition element    such as TiS₂ or FeS₂ and/or a vanadium oxide such as V₂O₅ and/or a    metal sulfide based on lithium of the “rock-salt” type such as    Li₂TiS₃ or Li₃NbS₄;-   when the lithium salt is lithium perchlorate, the positive electrode    advantageously comprises elementary sulfur as the active material;-   when the lithium salt is LiFSI, the positive electrode    advantageously comprises, as active material, a lithiated phosphate    comprising at least one metallic transition element such as LiFePO₄    and/or a disulfide based on at least one metallic transition element    such as TiS₂ or FeS₂ and/or a vanadium oxide such as V₂O₅ and/or a    metal sulfide based on lithium of the “rock-salt” type such as    Li₂TiS₃ or Li₃NbS₄ and/or a lithiated oxide comprising at least one    metallic element such as Li(Mn,Ni,Co)O₂;-   when the lithium salt is lithium triflate, the positive electrode    advantageously comprises, as an active material, elementary sulfur    and/or a disulfide based on at least one metallic transition element    such as TiS₂ or FeS₂ and/or a metal sulfide based on lithium of the    “rock-salt” type, such as Li₂TiS₃ or Li₃NbS₄;-   when the lithium salt is LiBETI, the positive electrode    advantageously comprises, as an active material, a lithiated    phosphate comprising at least one metallic transition element, such    as LiFePO₄ or LiMnPO₄ and/or elementary sulfur and/or a disulfide    based on at least one metallic transition element, such as TiS₂ or    FeS₂ and/or a vanadium oxide, such as V₂O₅ and/or a metal sulfide    based on lithium of the “rock-salt” type, such as Li₂TiS₃ or Li₃NbS₄    and/or a lithiated oxide comprising at least one metallic element    such as LiCoO₂, Li(Mn,Ni,Co)O₂ (such as    LiNi_(0,6)Mn_(0,2)Co_(0,2)O₂, LiNi_(0,8)Mn_(0,1)Co_(0,1)O₂),    Li(Ni,Co,Al)O₂ (such as LiNi_(0,8)Co_(0,15)Al_(0,05)O₂), LiMn₂O₄ or    an overlithiated oxide comprising manganese, nickel and possibly    cobalt such as oxides of formula Li_(1+x)(Mn,Ni,Co)O₂ (in which x>0,    for example x=0.2, such as Li_(1,2)Mn_(0,6)Ni_(0,2)O₂).

The compound comprising aluminium may be an aluminium salt and morespecifically an in situ formed aluminium salt. Even more specifically,the aluminium salt may be an aluminium salt having aluminium cations,which may originate from partial corrosion of the aluminium currentcollector associated with the positive electrode. The anions mayoriginate from those of the lithium salt of the electrolyte or adecomposition of the anions of the lithium salt of the electrolyte.

For example, the aluminium salt may be comprise, as anions, anionscomprising one or several halogen atoms such as fluorine and/or chlorineand more specifically anions comprising one or several fluorine atomssuch as imidide anions and even more specifically,(trifluoromethane)sulfonylimidide anions, particularly when the lithiumsalt used is LiTFSI; or a triflate anion, particularly when the lithiumsalt used is lithium triflate.

The compound comprising aluminium may also be an alloy of lithium andaluminium.

A specific cell conforming with the invention is a cell in which:

-   -   the positive electrode comprises, as an active material, a        lithiated phosphate comprising at least one metallic transition        element, such as LiFePO₄; and/or    -   the electrolyte is a liquid electrolyte comprising at least one        lithium imidide salt, such as LiTFSI and one or several        carbonate solvents, such as an EC/DMC mixture.

The cells according to the invention may be prepared or regenerated by aprocess comprising a step consisting of subjecting:

-   a cell comprising a negative electrode comprising, as active    material, metallic lithium; a positive electrode associated with an    aluminium current collector; and an electrolyte located between said    negative electrode and said positive electrode comprising at least    one lithium salt chosen from among lithium imidide, lithium    triflate, lithium perchlorate salts and mixtures thereof;-   to a corrosion potential of aluminium for a sufficiently long time    to obtain partial corrosion of the aluminium collector and the    concomitant formation of a layer comprising a compound comprising    aluminium on the face of the negative electrode in contact with the    electrolyte.

To fix this corrosion potential and its application duration, priortests can be carried out with a test cell satisfying the same specificfeatures as those to be subjected to the method, the prior testsconsisting of testing different pairs of potentials and duration untilthe pair is obtained that can produce the layer, production of thislayer possibly being followed by EDX spectroscopy. The temperature canalso be adjusted, knowing that the temperature criterion can have aninfluence on the quantity of corroded material and consequently on thequantity of layer deposited.

Advantageously, the corrosion potential is fixed at a value higher thenthe functioning voltage of the cell, examples of ranges of operatingvoltages being given for specific active material/lithium salt pairsshown in the table given later.

According to one particular embodiment, the cell subjected to the methoddoes not comprise a layer including a compound containing aluminium,which means that the method is used on a cell that has not yetfunctioned. As a variant, the cell on which the method is used maycomprise a degraded layer containing a compound comprising aluminium,which in the latter case means that the method can be a regenerationmethod and that it can be used with a cell that has already been used.Indeed, for a cell conforming with the invention that has already beencycled one or several times, a lithium layer may form on the surface ofthe layer comprising a compound containing aluminium, thus making itexpedient to refresh this layer using the above-mentioned method.

For example, when the cell comprises an electrolyte containing a lithiumsalt LiTFSI and the positive electrode comprises LiFePO₄ as activematerial, the corrosion potential can be fixed at a value equal to 4.5 Vvs Li⁺/Li and held for a duration of the order of 10 hours to obtain theformation of a layer containing aluminium.

Other examples are given in the following table, indicating thepotential to be applied for each specific positive electrode lithiumsalt/active material pair to obtain partial corrosion of the aluminiumnecessary for the formation of a layer comprising aluminium and theoperating voltage range for these pairs.

Operating Corrosion voltage Cathode active potential range Lithium saltmaterial (in V vs. Li⁺/Li (in V vs. Li⁺/Li) LiTFSI LiFePO₄ >4 2.5-3.7LiTFSI TiS₂ >4 1.5-3   LiTFSI FeS₂ >4 0.5-2   LiTFSI V₂O₅ >4 2-4 LiTFSILi₂TiS₃ >4 1.5-3   LiTFSI Li₃NbS₄ >4 1.5-3   LiClO₄ Elementary sulfur S₈4.5 1.5-3   LiFSI LiFePO₄ >4.5 2.5-3.7 LiFSI TiS₂ >4.5 1.5-3   LiFSIFeS₂ >4.5 0.5-2   LiFSI V₂O₅ >4.5 2-4 LiFSI Li₂TiS₃ >4.5 1.5-3   LiFSILi₃NbS₄ >4.5 1.5-3   LiFSI Li(Mn, Ni, Co)O₂ >4.5   3-4.3 Lithiumtriflate Elementary sulfur S₈ >3 1.5-2.5 Lithium triflate TiS₂ >31.5-2.5 Lithium triflate FeS₂ >3 0.5-2   Lithium triflate Li₂TiS₃ >31.5-2.5 Lithium triflate Li₃NbS₄ >3 1.5-2.5 LiBETI LiFePO₄ >4.5 2.5-3.7LiBETI Elementary sulfur S₈ >4.5 1.5-3   LiBETI TiS₂ >4.5 1.5-3   LiBETIFeS₂ >4.5 0.5-2   LiBETI V₂O₅ >4.5 2-4 LiBETI Li₂TiS₃ >4.5 1.5-3  LiBETI Li₃NbS₄ >4.5 1.5-3   LiBETI LiCoO₂ >4.5   3-4.2 LiBETI Li(Mn, Ni,Co)O₂ >4.5   3-4.2 LiBETI Li(Ni, Co, Al)O₂ >4.5 3.5-4.3 LiBETILiMnPO₄ >4.5 3.5-4.3 LiBETI LiMn₂O₄ >4.5 3.5-4.3 LiBETI Li_(1+x)(Mn, Ni,Co)O₂ >4.5 3.5-4.4

Finally, the invention relates to the use of a layer comprising acompound containing aluminium on a metallic lithium electrode, containedin a cell, in order to improve its cyclability, said cell comprising apositive electrode associated with an aluminium current collector and anelectrolyte arranged between the positive electrode and the negativeelectrode and containing at least one lithium salt chosen from amonglithium imidide, lithium triflate, lithium perchlorate salts andmixtures thereof, said layer comprising a compound containing aluminiumbeing deposited on the face of the negative electrode in contact withthe electrolyte.

The special features of the positive electrode, the electrolyte and thelayer already defined for cells as such are also valid for this use.

Other characteristics and advantages of the invention will become clearafter reading the following additional description and that applies toparticular embodiments.

Obviously, this additional description is only given to illustrate theinvention and in no way forms a limitation of it.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a sectional view of a specific cell according to theinvention;

FIG. 2 is a graph illustrating the variation of the potential U (in V)(left ordinate) as a function of time (in h) (curve a) and thesimultaneous variation of the current I (in A) (right ordinate) as afunction of time (in h) (curve b) for a specific cell conforming withthe invention.

FIG. 3 is a graph illustrating the variation of the potential U (in V)(left ordinate) as a function of time (in h) (curve a) and thesimultaneous variation of the current I (in A) (right ordinate) as afunction of time (in h) (curve b) for a specific cell not conformingwith the invention.

FIG. 4 is a graph illustrating the variation of the discharge capacity C(in Ah) as a function of the number N of cycles, (curve a) for aspecific cell conforming with the invention and curve b) for a specificcell not conforming with the invention-.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS EXAMPLE

This example illustrates an electrochemical cell conforming with theinvention and, for comparison, a cell not conforming with the invention(called first cell not conforming with the invention) so as todemonstrate the benefit of the layer comprising a compound containingaluminium on the lithium electrode, on the cyclability properties ofthis cell.

In this example, before the formation cycle, said electrochemical cellsare in the form of a button battery comprising a stack of layers asillustrated in appendix FIG. 1 comprising:

-   a negative electrode 1 composed of metallic lithium in the form of a    16 mm diameter 150 μm thick disk deposited on stainless steel    packing (not shown) that acts as a current collector;-   a positive electrode 3 comprising, as active material, LiFePO₄, as    conducting electricity material, carbon black and PVDF as polymeric    binder in proportions by mass of 90.5/5/4.5 respectively, said    positive electrode being deposited on an aluminium current collector    5 composed of a 20 μm thick disk with a surface area of 154 mm²; and-   a porous separator 7 composed of a 16.5 mm diameter and 25 μm thick    polyolefin disk, said separator being soaked with a liquid    electrolyte consisting, for the cell conforming with the invention,    of a mixture of carbonate solvents (ethylene carbonate/dimethyl    carbonate) and 1M LiTFSI and, for the cell not conforming with the    invention, of a mixture of carbonate solvents (ethylene    carbonate/dimethyl carbonate) and 1M LiPF₆.

Each of these cells is subjected to a formation cycle, particularly withthe application of a fixed potential level of 4.5V as commoncharacteristic (vs. Li⁺/Li) for a duration of 10 hours.

The characteristics of the formation cycle and the current generatedduring this cycle are shown in FIG. 2 for the cell conforming with theinvention, and on FIG. 3 for the cell not conforming with the invention,these figures illustrating firstly the variation of the potential U (inV) (left ordinate) as a function of the duration t (in h) (curves a) onFIG. 2 and FIG. 3) and the simultaneous variation of the current I (inA) (right ordinate) as a function of the duration in t (curves b) onFIG. 2 and FIG. 3).

For the cell conforming with the invention, during the 4.5 V plateau,the first observation was a reduction in the current, followed by anincrease representing a corrosion current indicating a partial corrosionphenomenon of the aluminium collector.

More specifically, during the plateau at 4.5V, a capacity of 0.94 mAh ischarged. Assuming the Al→Al³⁺+3 e⁻ corrosion reaction (3×96500 C per molof corroded aluminium, namely 80.4 Ah/mol), 1.17×10⁻⁵ mol of aluminiumwas corroded. This corresponds to 0.3 mg of aluminium, or 0.1 mm³ of Al.The surface area of the collector in the button battery is 154 mm². Itis deduced that about 0.7 μm of collector was corroded out of a total of20 μm, which does not jeopardise its electrical conduction phenomenon.

For the cell not conforming with the invention, during the 4.5 Vplateau, the first observation was a reduction in the current, followedby stabilisation of the current, which shows that there is no corrosion,this no corrosion continuing until at least 5 V.

After this formation cycle, each cell is subjected to galvanostaticcycling in a range of potentials varying from 2.5 to 3.7 V and a currentof 0.85 mA, the results being shown on the appended FIG. 4 illustratingthe variation of the discharge capacity C (in Ah) as a function of thenumber of cycles N (curve a) for the cell conforming with the inventionand curve b) for the cell not conforming with the invention).

A significant improvement in cyclability of the cell according to theinvention is found (with the capacity stabilising at at least 60 cycles)compared with the cell not conforming with the invention using LiPF₆ aslithium salt not having been subjected to a partial corrosion phenomenonof the aluminium collector.

The authors of this invention performed this galvanostatic cycling witha second series of cells (one cell conforming with the invention and onecell not conforming with the invention) satisfying the same specificfeatures as mentioned above and having been subjected to a formationcycle identical to that defined above, galvanostatic cycling beingstopped in this case at 6 cycles, with the objective of finding anexplanation for this phenomenon.

The cells were then opened and the lithium negative electrode wasanalysed by EDX spectroscopy.

For the cell conforming with the invention, the presence of a layercomprising a compound containing aluminium was found on the lithiumelectrolyte, the aluminium originating from partial corrosion of thecurrent collector.

There is no such layer on the surface of the lithium electrode of thecell not conforming with the invention, and in particular there is noaluminium on the surface of the lithium electrode.

It is thus deduced that the presence of this layer on the surface of thelithium electrode in contact with the liquid electrolyte containingLiTFSI contributes to improving cyclability of the cell conforming withthe invention.

Finally, to clearly demonstrate that the improvement in cyclability isnot simply due to the lithium salt used, cells not conforming with theinvention (more specifically a second cell not conforming with theinvention using as electrolyte, LiPF₆ in a mixture of EC/DMC carbonatesolvents and a third cell not conforming with the invention using, aselectrolyte, LiTSI in a mixture of EC/DMC carbonate solvents, thesenon-conforming cells being similar in other respects to the first cellnot conforming with the invention relating to positive and negativeelectrodes) were manufactured and cycled between 2.5 and 3.7 V withoutapplication of a plateau at 4.5 V (namely a plateau possibly enablingformation of the layer comprising aluminium on the surface of thenegative electrode). In both cases, cyclability is similar is similar tocyclability of the first cell not conforming with the invention.

1. Electrochemical cell for lithium accumulator comprising: a negativeelectrode comprising metallic lithium as active material; a positiveelectrode associated with an aluminium current collector; and a liquidor gel electrolyte placed between said negative electrode and saidpositive electrode, characterised in that the negative electrode isprovided at its face in contact with the electrolyte with a layercomprising a compound containing aluminium, said compound being analuminium salt, and in that the electrolyte comprises at least onelithium salt chosen from among lithium imidide, lithium triflate,lithium perchlorate salts and mixtures thereof.
 2. Cell according toclaim 1, wherein the positive electrode comprises as active material, anactive material chosen from among lithiated phosphates comprising atleast one metallic transition element, lithiated oxides comprising atleast one metallic element, lithium based metal sulfides, vanadiumoxides, disulfides based on at least one metallic transition element,elementary sulfur and mixtures thereof.
 3. Cell according to claim 2wherein, when the active material is a lithiated phosphate comprising atleast one metallic transition element, it corresponds to a lithiatedphosphate with formula LiM¹PO₄, wherein M¹ is chosen from among Fe, Mn,Ni, Co and mixtures thereof.
 4. Cell according to claim 2 wherein, whenthe active material is a lithiated oxide comprising at least onemetallic transition element, it corresponds to a lithiated oxidecomprising nickel, cobalt, manganese and/or aluminium.
 5. Cell accordingto claim 4, wherein the lithiated oxide comprising nickel, cobalt,manganese and/or aluminium satisfies the following formula:Li_(i+x)M²O₂ wherein M² is an element chosen from among Ni, Co, Mn, Aland mixtures thereof and x is greater than or equal to
 0. 6. Cellaccording to claim 2 wherein, when the active material is a lithiumbased metal sulfide, it corresponds to a lithium based metal sulfide ofthe “rock-salt” type, such as Li₂TiS₃ or Li₃NbS₄.
 7. Cell according toclaim 2 wherein, when the active material is a vanadium oxide, itcorresponds to vanadium oxide V₂O₅.
 8. Cell according to claim 2wherein, when the active material is a disulfide based on at least onemetallic transition element, it corresponds to a crystallising disulfidein a pyrite type crystalline system, such as FeS₂, TiS₂.
 9. Cellaccording to claim 1, wherein the liquid electrolyte comprises one orseveral organic solvents in the carbonate solvents family and/or theether solvents family.
 10. Cell according to claim 1, wherein the gelelectrolyte is a liquid electrolyte impregnating a polymer matrix thatgelifies in contact with the liquid electrolyte.
 11. Cell according toclaim 10, wherein the liquid electrolyte impregnating the polymer matrixis a liquid electrolyte as defined in claim
 9. 12. Cell according to anyone of the preceding claims wherein, when the lithium salts are lithiumimidide salts, they are chosen from among lithiumbis(trifluoromethane)sulfonylimidide with formula Li[N(SO₂CF₃)₂),lithium bis(fluorosulfonyl)imidide with formula Li[N(SO₂F)₂] and lithiumbis(pentafluoroethane)sulfonylimidide with formula Li[N(SO₂CF₂CF₃)₂].13. Cell according to claim 1, wherein: when the lithium salt is lithiumbis(trifluoromethane)sulfonylimidide, the positive electrode comprises,as active material, a lithiated phosphate comprising at least onemetallic transition element, such as LiFePO₄ and/or a disulfide based onat least one metallic transition element, such as TiS₂ or FeS₂ and/or avanadium oxide, such as V₂O₅ and/or a metal sulfide based on lithium ofthe “rock-salt” type, such as Li₂TiS₃ or Li₃NbS₄; when the lithium saltis lithium perchlorate, the positive electrode comprises, as activematerial, elementary sulfur; when the lithium salt is lithiumbis(fluorosulfonyl)imidide, the positive electrode comprises, as activematerial, a lithiated phosphate comprising at least one metallictransition element, such as LiFePO₄ and/or a disulfide based on at leastone metallic transition element, such as TiS₂ or FeS₂ and/or a vanadiumoxide, such as V₂O₅ and/or a metal sulfide based on lithium of the“rock-salt” type, such as Li₂TiS₃ or Li₃NbS₄ and/or a lithiated oxidecomprising at least one metal element such as Li(Mn,Ni,Co)O₂; when thelithium salt is lithium triflate, the positive electrode comprises, asactive material, elementary sulfur and/or a disulfide based on at leastone metallic transition element, such as TiS₂ or FeS₂ and/or a metalsulfide based on lithium of the “rock-salt” type, such as Li₂TiS₃ orLi₃NbS₄; * when the lithium salt is LiBETI, the positive electrodecomprises, as active material, a lithiated phosphate comprising at leastone metallic transition element, such as LiFePO₄ or LiMnPO₄ and/orelementary sulfur and/or a disulfide based on at least one metallictransition element, such as TiS₂ or FeS₂ and/or a vanadium oxide such asV₂O₅ and/or a metal sulfide based on lithium of the “rock-salt” type,such as Li₂TiS₃ or Li₃NbS₄ and/or a lithiated oxide comprising at leastone metallic element such as LiCoO₂, Li(Mn,Ni,Co)O₂, Li(Ni,Co,Al)O₂,LiMn₂O₄ or an overlithiated oxide comprising manganese, nickel andpossibly cobalt.
 14. Cell according to claim 1, wherein the aluminiumsalt comprises, as anions, anions comprising one or more several halogenatoms, such as fluorine and/or chlorine.
 15. Method of preparing orregenerating a cell as defined in claim 1, comprising a step consistingof subjecting: a cell comprising a negative electrode comprisingmetallic lithium as active material; a positive electrode associatedwith an aluminium current collector; and an electrolyte located betweensaid negative electrode and said positive electrode comprising at leastone lithium salt chosen from among lithium imidide, lithium triflate,lithium perchlorate salts and mixtures thereof; to a corrosion potentialof aluminium for a sufficiently long time to obtain partial corrosion ofthe aluminium collector and the concomitant formation of a layercomprising a compound containing aluminium on the face of the negativeelectrode in contact with the electrolyte.
 16. Use of a layer comprisinga compound containing aluminium, which is an aluminium salt, on ametallic lithium electrode, contained in a cell, in order to improve itscyclability, said cell comprising a positive electrode associated withan aluminium current collector and a liquid or gel electrolyte arrangedbetween the positive electrode and the negative electrode and containingat least one lithium salt chosen from among lithium imidide, lithiumtriflate, lithium perchlorate salts and mixtures thereof, said layercomprising a compound containing aluminium being deposited on the faceof the negative electrode in contact with the electrolyte.